This invention relates to a method of providing metallic material with an electrodeposited coating for use in working, such as pressing, stamping etc.
It is the recent trend that metallic material, particularly steel sheet to be worked through pressing, stamping and so on is coated with a lubricative coating by means of dip coating, etc., prior to being subjected to working. Such coating serves to lubricate the metal surface while it is subjected to forming. Such coating is also effective to keep metallic material, particularly metal sheets in a stack from slipping and falling down. Many processes for preparing such coating, therefore, have been proposed and are in development.
For use in working metallic materials, lubricants such as mineral oils, animal oils, vegetable oils and synthetic lubricants (e.g., synthetic esters) with or without an oiliness improver, an extreme pressure additive, etc. have been known in the art. However, they all have a disadvantage in that galling or scoring of the pressed articles is inevitable during working.
Soda soap and water soluble soap which comprises sodium borate or the like as its major component are also used as a lubricant. However, it is sometimes difficult to cover the newly developed metal surface completely with them during working, and incomplete covering leads to pressing flaws. Thus, a metal blank with such lubricant has less formability.
Japanese Patent Publication No. 22948/1977 discloses a lubricant comprised of a mixture of a resin component with wax or fatty esters, etc. It is rather difficult to apply this type of lubricant in a thin and uniform layer. However, if applied in a thick layer, the lubricant easily adheres to the surface of the die or punch and forms deposits thereon.
Japanese Patent Publication No. 29819/1980 discloses a dual-layer lubricative coating in which a liquid lubricant (fats and oils, for example) or a solid lubricant (graphite, for example) is applied over an organic resin film which has been previously placed on the metal substrate. However, this dual-layer coating type lubricant is expensive, since it is necessary to carry out hot-air drying and then bake-hardening at a temperature of 100.degree. C. or higher after the application thereof. In addition, a coating of fats and oils develops an unpleasant odor and the coated surface becomes sticky and rough.
Many methods for application of coating are known to those skilled in the art, including brushing, roll coating, dip coating, spray coating, curtain flow coating and so on. All these prior art processes, however, have a common disadvantage in that it is quite difficult to control application of the lubricant precisely so as to make the coating thin and uniform.
Usually, therefore, it is preferable to employ an electrodeposition coating process for such purpose. According to the electrodeposition coating process, a shearing stress is applied to the surface of the metal being treated, since the speed at which the steel sheet travels through the electrodeposition bath is very high. In addition, when the composition of the coating is comprised of a mixture of two or more components such as a resin and a lubricant which have different electrophoresis properties, it is sometimes difficult to obtain a coating having the intended properties, because the composition (i.e., resin-to-lubricant ratio) of the resulting coating differs much from that of the electrodeposition bath. This difference in composition between the electrodeposition bath and the coating becomes greater when such strong shearing stress as mentioned above is applied to the surface of the metal.